The morphologic features of 220 patients with hypertrophic cardiomyopathy (HC) were described. The major gross morphologic features of HC, in order of decreasing frequency, are as follows: 1 ) dilated atria (100%); 2) increased heart weight (95%); 3) nondilated left ventricle (79%); 4) thickened mitral valve (75%); 5) fibrous endocardial mural plaque, left ventricular outflow tract (72%); and 6) a ventricular septum thicker than the left ventricular free wall (60%). Gross left ventricular fibrosis occurred in 84% of the patients and rarely was a consequence of associated narrowing of an epicardial coronary artery disease. HC may cause death in any decade of life. The diagnosis of HC in the very young (age <10 years) and in the very old (age >70) is more difficult clinically and at necropsy than in the intermediate age groups. Patients with the obstructive type of HC usually have larger hearts, a higher frequency of a fibrous plaque in the left ventricular outflow tract, and a higher frequency of a thickened mitral valve than those without obstruction. Left ventricular cavity dilation is present in about 20% of patients with HC and is a result of either operation (with survival >43 months) or extensive left ventricular wall scarring.